Aging bolts are a possible culprit behind the collapse of concrete ceiling slabs that fell from the roof of a highway tunnel in Japan Monday, crushing three moving vehicles, killing nine people and injuring at least two others, USA Today reported[2].

The Sasago Tunnel is 3 miles long and links the capital to central Japan. An inspection of the of the tunnel’s roof in September showed no problems, Noguchi said.

Both sections of the highway, including a parallel tunnel for traffic going in the opposite direction, were shut down indefinitely. Following the collapse in the tunnel, the transport ministry called for inspections on 49 other tunnels across the country that are either on highways or roads managed by the central government and of similar construction.

An operator said it is possible that the bolts holding a metal piece suspending the panels above the road had aged. The 16-foot long, 4-foot wide, 3-inch thick panels have been in place since 1977, when the tunnel was constructed.

Recovery work at the tunnel was suspended Monday while the roof was reinforced to prevent more collapses, Jun Goto, an official at the Fire and Disaster Management Agency, told the AP. Goto said recovery work will hopefully resume on Tuesday.